1. Field of the Invention
The present invention relates to a thermal transfer sheet, in which a specific fatty acid ester compound is used for a heat-resistant lubricating layer. In particular, the present invention relates to a thermal transfer sheet excellent in running smoothness during transfer and preservation stability of dye.
2. Description of the Related Art
A thermal transfer system by using a sublimation dye transfers a large number of color dots to a transfer receiver through a very short time heating so as to reproduce a full color image based on the color dots of a plurality of colors.
In this thermal transfer system, a so-called sublimation thermal transfer sheet, in which a dye layer composed of a sublimation dye and a binder is disposed on one surface of a base material sheet, e.g., a polyester film, is used as a thermal transfer sheet.
In the thermal transfer system, a thermal transfer sheet is heated from the back with a thermal head in accordance with image information so as to transfer a dye contained in a dye layer to a transfer receiver (photographic paper) and, thereby, form an image.
At this time, regarding the thermal transfer sheet, it is desired that a surface on the side coming into contact with the thermal head stably exhibits low friction over low density image printing to high density image printing. In general, the thermal transfer sheet is provided with a heat-resistant lubricating layer on the surface opposite to the surface, on which the dye layer is disposed, in order to prevent fusion with the thermal head and give smooth running smoothness.
Incidentally, in image printing on the photographic paper by using a thermal transfer sheet, heat is applied to the heat-resistant lubricating layer from the thermal head and, thereby, a dye in the dye layer on the opposite surface is transferred to the photographic paper. The color formation density is proportionate to an amount of heat, and the surface temperature of the thermal head changes by a few hundreds of degrees, correspondingly. Consequently, when the thermal transfer sheet moves on the thermal head, the friction coefficient between the thermal head and the heat-resistant lubricating layer changes easily because of the temperature change. If the friction coefficient between the thermal head and the heat-resistant lubricating layer changes, movement of the thermal transfer sheet at a constant speed becomes difficult and, thereby, it is difficult to obtain a sharp image.
For example, in the case where the friction coefficient between the thermal head and the heat-resistant lubricating layer is large, movement of the thermal transfer sheet becomes slow temporarily, and the density of merely that portion may become high. That is, so-called sticking (linear variations in image printing) may occur.
In order to prevent this sticking, it is desirable that the friction coefficient at, in particular, high temperatures is reduced. As for lubricants to reduce the friction coefficient at high temperatures, phosphate esters and fatty acid esters have been used previously, and the phosphate esters and the fatty acid esters have been contained in the heat-resistant lubricating layers (refer to Japanese Unexamined Patent Application Publication No. 10-35122, for example).
However, the phosphate esters and the fatty acid esters, which are used frequently in general, are volatilized or decomposed by heat from the thermal head so as to stain the thermal head. If image printing is further conducted repeatedly with this stained thermal head, adhered materials are baked on the thermal head surface, and the heat of the thermal head is not conducted to the thermal transfer sheet appropriately because of the baked adhered materials. As a result, variations in image printing and the like occur in the image printing.
Furthermore, in the case where the thermal transfer sheet is preserved in a rolled state, contact between the dye layer and the heat-resistant lubricating layer occurs. Therefore, in particular in a state of high temperature preservation, the phosphate esters and the fatty acid esters having low melting points and high solvency dissolve a part of dye in the dye layer. Consequently, reduction in print image density, variations in image printing, and the like occur in the image printing by using the thermal transfer sheet, from which the dye has been eluted.
As for the lubricant to reduce the friction coefficient, silicone oils are used (refer to Japanese Unexamined Patent Application Publication No. 04-329193, for example).
Regarding the thermal transfer sheet including the silicone oil as well, since the silicone oil is a liquid at ambient temperature, in the case where the thermal transfer sheet is preserved in a rolled state, contact between the dye layer and the heat-resistant lubricating layer occurs, so that a part of dye in the dye layer is eluted. Consequently, reduction in density in the image printing, variations in image printing, and the like occur.